I believe they were planning to make a mining stage but they removed from the list and it was half complete. so instead of throwing away their half-complete work, they merged it with some parts of Valis 3 to make a complete stage as a secret stage (some ideas, IMO but since I'm a developer myself, I'm quite sure thats what they did )

It kinda looked like one of those stages leading up to an entrance of some sort, like it was the supply route to Galagher's (sp?) palace. I've not played or really looked much into Valis IV, so I don't know exactly how they reached Galagher. However, it does indeed look like an unfinished stage that they just tacked something to the end of and used it as an easter egg.